1. Field of the Invention
The present invention relates to an optical information recording medium, more particularly to a write-once-type compact disc having a ROM area and a recordable area, and a method of fabricating the write-once-type compact disc.
2. Discussion of Background
Recently, development of write-once-type compact discs (CDs) has advanced actively. Unlike the conventional CDs, information can be recorded in the write-once-type CDs by the user. Moreover, the write-once-type CDs are characterized in that the recorded information can be reproduced by a commercially available CD player because the recording signals for the write-once-type CDs conform to the conventional CD standards.
Japanese Laid-Open Patent Application No. 2-42652 discloses one method of fabricating such a write-once-type CD in which a light absorption layer is formed on a substrate by coating dye on the substrate by the spin coating method and in which a metallic reflection layer is provided on the above formed light absorption layer.
Japanese Laid-Open Patent Application No. 2-132656 describes a write-once-type CD for which recorded signals conforming to the conventional CD standards can be fabricated by adequately adjusting the complex index of reflection of the light absorption layer and the thickness thereof.
Such a write-once-type CD can be utilized in such a fashion to include (a) a ROM area in which information pits are preformed and (b) a recordable area in which information is to be recorded later as desired by the user.
In order to form such a ROM area in the write-once-type CD, the following two methods can be employed:
(i) A CD with the entire surface thereof being recordable, which consists of a recordable ROM area and another recordable area is fabricated. The manufacturer of the CD provides the user with the necessary information recorded in the ROM area before sale.
(ii) Pits for recording information are formed in an area corresponding to a ROM area of a CD in a step for producing a substrate thereof in the same manner as in the conventional CDs, and then the remaining area in which pits are not formed is provided as a recordable area that can be used by the user.
The previously mentioned Japanese Laid-Open Patent Application No. 2-42652 discloses a CD, which includes a ROM area 5 and a recordable area 6 formed by the above-mentioned method (ii) as shown in FIG. 2A and FIG. 2B. This CD is composed of a transparent substrate 11, a light absorption layer 12 comprising a dye formed in the recordable area 6 on the substrate 11, a reflection layer 13 formed not only in the ROM area 5, but also on the light absorption layer 12, and a protective layer 14 which covers the light reflection layer 13. In this CD, the light absorption layer 12 is provided only in the recordable area 6, not in the ROM area 5. In other words, the "separate coating" method is employed when the recordable area of the CD is coated with the dye.
The reasons for employing the separate coating method are considered as follows: When the surface of a substrate 21 having pits or grooves is coated with the dye composition using the spin coating method, the pits or the grooves are generally filled with the dye composition in such a form as shown in FIG. 3A, in which the thickness of the coated dye composition layer 22a in the bottom portion of the pit is much thicker than that of the coated dye composition layer 22a in the land portion of the pit. In other words, it is extremely difficult to uniformly coat the pit with the dye composition in the form of a dye composition layer 22b as shown in FIG. 3B.
In particular, pits formed on the surface of the ROM area are substantially the same as those formed on conventional CDs, and therefore, the signals which conform to the CD standards can be obtained when a reflection layer is directly formed on the ROM area. However, if the reflection layer is provided after coating the ROM area with the dye composition in such a fashion as shown in FIG. 3A, the shapes of the pits are substantially changed, the modulation degree of the recording medium is reduced, that is, the amplitude of signals is substantially decreased. The result is that the signals conforming to the CD standards cannot be obtained.
Thus, the separate coating method is employed in Japanese Laid-Open Patent Application No. 2-42652.
However, such a separate coating method has the following shortcomings:
(1) Although the light absorption layer is generally formed by the spin coating method, it is practically impossible to strictly coat only the recordable area with a dye composition, without adverse effects to the ROM area from the dye composition. Therefore, as a matter of fact, it occurs that part of the ROM area is coated with the dye composition or part of the recordable area is not coated with the dye composition. As a result, these improperly coated or uncoated parts cause improper function of the recording medium.
(2) The coated light absorption layer tends to become thicker in a portion close to the border with the ROM area than in the other portion, so that the reflectance of the light beam is not up to the CD standards. Such a portion does not properly function as the light absorption layer and cannot be used. Furthermore, such unusable portions make up a large area because the dye composition cannot always be uniformly coated onto the substrate. This leads to a substantial decrease in the recording capacity of the CD.
The substrate of the optical information recording medium, with preformed pits and grooves, is generally formed by injection molding, using a thermoplastic resin such as polycarbonate or polymethyl methacrylate. When a dye composition is dispersed in a solvent and coated onto such a substrate by the spin coating method, the solvent contained in the dye dispersion, depending on the kind of the solvent, may dissolve the surface of the substrate. If this occurs, the pits or the grooves are caused to disappear and the light reflectance of the substrate significantly decreases. Therefore, it is necessary to use a solvent which does not dissolve the substrate material, but dissolves the dye.
Japanese Laid-Open Patent Application No. 63-159090 discloses a fluorinated alcohol which can be employed as such a solvent that does not dissolve the substrate but dissolves the dye. The solvent containing the fluorinated alcohol has excellent properties as a coating liquid for the formation of the light absorption layer. However, when such a coating liquid is used, the pits or the grooves are filled with the dye composition as illustrated in FIG. 3A.